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Unit 5: Reactivity

Alloys — HIA International - Grade 8 Revision Notes & Topical Questions

5 min read 7 parts Lesson 4 of 12 2 quizzes

📚 HIA International - Grade 8 Hub / Topical Revision
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Lesson summary

Alloys — HIA INTERNATIONAL notes: Explain what alloys are, why they are harder than pure metals and give uses.Alloy = a mixture of metals, made by mixing metals together and melting them.Not compounds — the atoms mix but do not bond together, and the proportions can vary.Properti…

  • Properties
  • Layers stuck
  • Aeroplanes
  • Artificial joints
  • Steel — unusual because it is a mixture of iron and carbon, a non-metal
  • Alloy — atoms of a different size disrupt the regular pattern

Section 1 of 7What an alloy is

Alloy
Alloy a mixture of metals, made by mixing metals together and melting them.
Not compounds
  • Not compounds the atoms mix but do not bond together, and the proportions can vary.
Properties
  • Properties they are different from the metals they contain.
Steel
  • Steel unusual because it is a mixture of iron and carbon, a non-metal.

Section 2 of 7Why alloys are harder

Pure metal
  • Pure metal atoms are all the same size in regular rows, so the layers slide over each other easily when hit or stretched.
Alloy
  • Alloy atoms of a different size disrupt the regular pattern.
Layers stuck
  • Layers stuck the layers cannot slide over each other easily.
Result
  • Result the alloy is harder and stronger than the pure metal.
1
Make rows of equal bubbles in a dish of diluted washing-up liquid to model a pure metal.
2
Observe how easily the rows slide past one another.
3
Inject one larger bubble into the middle to model an alloy atom.
4
Observe how the regular pattern is disrupted.

Section 3 of 7Common alloys

AlloyMade fromProperties and uses
SteelIron + carbonHarder than iron; bridges, buildings, cars
Stainless steelIron + chromium and nickelDoes not rust; cutlery
BronzeCopper + tinHarder than either metal; statues, bells, ship fittings
BrassCopper + zincHard, does not corrode, golden; musical instruments, plumbing
DuraluminAluminium + magnesium + copperAbout five times stronger than aluminium; aeroplanes
NitinolNickel + titaniumShape memory alloy; glasses frames
SolderTin + leadLow melting point; joins electrical parts

Section 4 of 7Alloys in everyday life

Coins
  • Coins pure metals are too soft, so coins are alloys;
    • silver-coloured coins use copper and nickel, copper-coloured use copper, zinc and tin.
Jewellery
  • Jewellery pure gold is soft and is 24 carat; 18 carat has 18 parts gold and 6 parts other metals.
Aeroplanes
  • Aeroplanes they need a light, very strong metal, so aluminium alloys are used.
Artificial joints
  • Artificial joints alloys, often of titanium, are used for replacement joints.

Section 5 of 7Melting points of alloys

Pure metals
  • Pure metals have one melting point, e.g. copper 1085 °C, zinc 419.5 °C and tin 232 °C.
Alloys
  • Alloys have a range of melting points, because the proportions of metals can vary.
Brass
  • Brass melts at about 900–1000 °C.
Bronze
  • Bronze melts at about 860–1150 °C.

Section 6 of 7Key terms

TermMeaning
AlloyA mixture of metals, or of a metal with other elements.
SteelAn alloy of iron and carbon.
BronzeAn alloy of copper and tin.
BrassAn alloy of copper and zinc.
CaratA measure of the purity of gold; 24 carat is pure.
DisruptTo break up a regular pattern.

Section 7 of 7Exam technique

1
Alloys are compounds — they are mixtures with no chemical bonds between the metals.
2
Alloys are made of non-metals — most are metal mixtures, though steel contains carbon.
3
Pure silver is used for coins — it is too soft, so alloys are used.
4
An alloy has a fixed melting point — it melts over a range depending on its proportions.

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